In recent years, recordable optical disks which allow a large amount of information to be recorded thereon, and optical disk apparatuses which are compatible with them, are becoming prevalent. FIG. 12 schematically shows the structure of a conventional optical disk apparatus which only performs reproduction or a conventional optical disk apparatus which performs recording and reproduction. Since an optical disk apparatus which performs recording is generally also capable of reproduction, an optical disk apparatus which performs recording and reproduction will simply be referred to as an optical disk apparatus which performs recording, in the present specification.
In the conventional optical disk apparatus shown in FIG. 12, light emitted from a laser 111 is converged by a collimating lens 102 so as to take a predetermined convergence state, and enters a polarization beam splitter 103 (which may also be abbreviated as PBS). The polarization beam splitter 103 reflects the incident light so that a portion thereof will enter a frontlight detector 112. Most of the incident light is transmitted through the polarization beam splitter, and enters a quarter-wave plate 104, where the polarization direction of the incident light is converted from linear polarization to circular polarization.
On a recording layer of an optical disk 101 which is rotated by a spindle motor 107, the light which has been transmitted through the quarter-wave plate 104 is converged by an objective lens 105, which is driven by an actuator 106, so as to take a predetermined convergence state.
The light which has been converged on the recording layer of the optical disk 101 is reflected from the recording layer, so that the reflected light enters the quarter-wave plate 104 via the objective lens 105. The quarter-wave plate 104 converts the polarization direction of the reflected light from circular polarization to linear polarization. This polarization direction is perpendicular to the polarization direction of the light which is emitted from the laser 111, transmitted through the polarization beam splitter 103, and travels toward the quarter-wave plate 104.
The light which has been transmitted through the quarter-wave plate 104 enters the polarization beam splitter 103. As described above, this light is perpendicular to the polarization direction of the light which is allowed to be transmitted through the polarization beam splitter 103, and therefore is not transmitted through to the laser 111 side, but is reflected toward a photodetector 113.
FIG. 13A and FIG. 13B respectively show the general constitution of a light source driving section 120 and a signal processing section 121 which are connected to the frontlight detector 112 and the photodetector 113.
As shown in FIG. 13A, the light which is received by the frontlight detector 112 is converted into an electrical signal, and is output as a frontlight signal to the light source driving section 120. Based on the frontlight signal, the light source driving section 120 drives the laser 111 in such a manner that the laser light emitted from the laser 111 has a constant output power. For this purpose, the light source driving section 120 includes a laser power controller (hereinafter abbreviated as LPC) 114 and a high-frequency module (hereinafter abbreviated as HFM) 118. The LPC 114 extracts a low-frequency component from the frontlight signal, and controls a driving current for driving the laser 111 so that the low-frequency component of the frontlight signal stays constant. The HFM 118 subjects the driving current received from the LPC 114 to a high-frequency modulation, so that the laser 111 is driven with the modulated driving current.
On the other hand, as shown in FIG. 13B, the light received by the photodetector 113 is converted into an electrical signal, and input to the signal processing section 121 as an RF signal. The signal processing section 121 includes a servo control section 117 and an RF detection section 116, and the RF signal is input to the servo control section 117 and the RF detection section 116. Based on the RF signal, the servo control section 117 generates a focusing signal, a tracking signal, and the like for moving the objective lens along the focusing direction and the tracking direction. From the RF signal, the RF detection section 116 generates a reproduced signal, which contains the user information, address information, and the like recorded on the optical disk 101.
The polarization directions of the quarter-wave plate 104 and the polarization beam splitter 103 are designed so that almost all of the reflected light from the optical disk 101 enters the photodetector 113. In practice, however, due to variation in the amount of birefringence at a substrate which is provided on the recording layer surface of the optical disk 101, variation in the optical characteristics and adjustment of the quarter-wave plate 104, the polarization beam splitter 103, and the like, and fluctuation and variation in the wavelength of the laser 111, etc., the polarization direction of the polarization beam splitter 103 and the polarization direction of the reflected light are not completely perpendicular, so that there will be some light entering the laser 111 in an actual optical disk apparatus. This light is referred to as “returned light”.
In general, returned light increases as the light emitted from the laser 111 increases. However, depending on the phase difference between the light emitted from the laser 111 and the reflected light from the optical disk 101, the reflected light may be weakened due to interference with the light emitted from the laser 111. In this case, conversely, returned light will decrease as the light emitted from the laser 111 increases. The returned light to the laser 111 is absorbed in a semiconductor chip of the laser 111, thus contributing to the resonation of the laser 111, i.e., emission. For this reason, the laser emission efficiency increases in the presence of returned light, whereby the output power is increased.
FIG. 14 is a graph showing relationships between the driving current and the output power of a laser. In the figure, a solid line 61 shows a relationship in the case where the light amount of the returned light to the laser is small, whereas a broken line 62 shows a relationship in the case where the light amount of the returned light to the laser is large.
Via control of the LPC 114 utilizing the frontlight signal as described above, the output power of the laser 111 is adjusted so as to be constant. Herein, output power means the outgoing light amount from the laser.
When the optical disk apparatus reproduces information which is recorded on the optical disk 101, the light amount of the reflected light rapidly changes while tracing on the recording marks, pits, spaces, and the like which are formed on the optical disk 101. As a result, the light amount of the returned light to the laser 111 also changes.
However, since the changes in the light amount of the reflected light due to recording marks and spaces occur sufficiently faster than the control of the LPC 114, if a state in which there is a small amount of returned light (a white circle 63 in the graph) transitions to a state where there is a large amount of returned light (a black circle 64 in the graph) due to a change in the light amount of the reflected light, the output power will change within a range sandwiched by the two arrows. In other words, although the current which drives the laser 111 does not change, the output power will change, that is, the emission efficiency will change. Such fluctuations in the output power will hereinafter be referred to as scoop.
FIG. 15A and FIG. 15B show relationships between recording marks, an RF signal, and a frontlight signal. As shown in FIG. 15A, when recording marks 131 and a space 132 are disposed on a recording track 130 of an optical disk as in the figure, reflectance will decrease at the recording marks 131. However, in the absence of scoop, the frontlight signal 134 stays constant. In other words, the output power of the laser 111 does not change. As a result, as shown in the figure, an RF signal 133 having a proper waveform is obtained.
On the other hand, as shown in FIG. 15B, if the light amount of the returned light increases, the output power of the laser 111 fluctuates in accordance with the change in the light amount of the returned light, thus causing scoop. As a result, the frontlight signal 136 also fluctuates. Since fluctuation in the frontlight signal 136 due to scoop occurs sufficiently faster than the control speed of the LPC 114, the LPC 114 does not control the driving current for the laser 111 in response to the fluctuation in the frontlight signal 136. Therefore, to the RF signal 135, not only the fluctuation due to a change in the reflectance and phase of the recording marks 131 and the space 132, but also the fluctuation in the emission efficiency due to scoop is applied. For example, when the peak intensity of the RF signal 135 is used as a reference, due to a decrease in reflectance as well as a decrease in emission efficiency, the intensity becomes smaller in the regions of the recording marks 131. Therefore, the asymmetry and degree of modulation will shift relative to those of the RF signal 133 in FIG. 15A. As a result, the quality of the RF signal is degraded, and the reproduction jitter and error rate are deteriorated.
When performing a recording to an optical disk on an optical disk apparatus which performs recording and reproduction, it is necessary to form recording mark which satisfy a predetermined standard or reference, in order to guarantee a compatibility such that the recorded optical disk will permit correct reproduction also on another optical disk apparatus. Therefore, on such an optical disk apparatus, a predetermined recording pattern is first recorded onto the optical disk. Then, the recorded marks are irradiated with light, and the asymmetry and degree of modulation of an RF signal which is obtained through reproduction are evaluated. Based on the evaluation result, the optical disk apparatus adjusts the laser power used for recording so that the formed recording marks satisfy a predetermined standard or reference.
At this learning, if the laser output power has fluctuations due to scoop, the RF signal obtained from recording marks which are formed in the aforementioned manner cannot be correctly evaluated. If asymmetry is lost, a write compensation learning: for adjusting for edge shifts at the front end and rear end of a recording mark can no longer be performed accurately, either.
In order to reduce such scoop which exerts unfavorable influences on RF signal detection, for example, Japanese Laid-Open Patent Publication No. 2001-189028 proposes increasing the reflectance on the outgoing face of the laser in order to reduce the amount of returned light to the laser. Moreover, Japanese Laid-Open Patent Publication No. 2001-143299 discloses increasing reproduction power to suppress noise when a jitter due to scoop increases during reproduction of an optical disk. Moreover, Japanese Laid-Open Patent Publication No. 05-217193 proposes varying the oscillation frequency and duty of an HFM based on the reproduced radius of the optical disk, thus suppressing scoop.
However, according to the method of Japanese Laid-Open Patent Publication No. 2001-189028, the returned light may rather increase in the case where the reflectance of the optical disk is greater than that of the outgoing end face of the laser, thus deteriorating the reproduction jitter and error rate. Moreover, according to the method of Japanese Laid-Open Patent Publication No. 2001-143299, it is necessary to measure the jitter, and sufficient effects cannot be obtained in the case where the reproduced signal has a poor signal quality.
Moreover, the light amount of the returned light to the laser 111 also changes due to causes other than tracing on recording marks, pits, spaces, and the like formed on the optical disk 101. For example, if the optical disk 101 which is under reproduction is warped, the distance between the laser 111 and the recording layer of the optical disk 101 will fluctuate. Therefore, the phase difference between the light emitted from the laser 111 and the reflected light from the optical disk 101 will fluctuate, thus resulting in changes in the intensity of the returned light due to light interference. Such fluctuations in the returned light also cause scoop, whereby the RF signal quality will be degraded and the reproduction jitter and error rate will be deteriorated.